A movement unit is driven by a feed motor controlled by a NC machine. The NC machine controls a position and a speed of the movement unit and prevents an excessive load from being applied to the movement unit by restricting a torque of a feed motor if necessary.
In general, a multi-axis control is performed in accordance with a recent development of a control technology. Further, an unmanned processing operation for a long period of time is also generally performed by an integral control of a machine tool and a conveying device such as a work load or unloader.
When an automation of the machine tool is developed, an accident such as a collision of the movement unit with respect to the other member easily occurs due to a control error or a supply error of the work. Therefore, when a collision accident occurs, a control of detecting an overload acting on the movement unit and emergently stopping the movement unit is performed in order to minimize damage to the work or machine.
For example, a tool post 4a of a lathe illustrated in FIG. 6 moves in the X and Z directions by the rotation of an X-axis feed motor 5x and a Z-axis feed motor 5z and a work w gripped by a spindle chuck 7 is processed by a tool 6 attached to the tool post. The tool post 4a moves at a low speed during a processing operation in response to a speed instruction e from the NC machine to the X-axis motor 5x and the Z-axis motor 5z and moves at a high speed when moving to the next position after the processing operation on one block ends.
A work loader illustrated in FIG. 7 loads a material work w gripped by a work hand 4b of a tip of an arm 8 into the spindle chuck 7 of the lathe. The work hand 4b moves at a high speed u from a standby position indicated by an imaginary line in FIG. 7 to a position near the spindle chuck 7 and moves at a low speed v in the Z direction to insert the material work w into the spindle chuck 7.
There is a case where the tool post 4a or the tool 6 in a moving state collides with the spindle chuck 7 or the work w due to an erroneous processing program or a wrong material work. In order to prevent damage of the machine or the work due to a collision, a collision detecting apparatus which emergently stops the feed motor 5 by the detection of a collision is provided.
A Z-direction emergency stop device of FIG. 6 will be described. A current meter 18 is provided in a power supply circuit of a Z-axis feed motor 5z. When the moving tool 6 collides with the work w or the chuck 7, a current of the motor 5z increases due to an overload generated by the collision. When the current exceeds a value set by a setter 35, an emergency stop signal a is output from a comparator 31 so that the feed motor 5z is emergently stopped. Although not illustrated in the drawings, the same emergency stop device is also provided in the X-axis feed motor 5x. 
The collision detecting apparatus which electrically detects the overload can easily change a setting value and the machine can be easily recovered to a normal state after the emergency stop. Further, when the feed motor 5 is slightly rotated reversely after the emergency stop, a stress generated in the movement unit or the colliding member due to the collision is released and thus damage to the machine or the work can be reduced.
In Patent Document 1, the present applicant proposes a method in which a plurality of collision detection setting loads are set in a load setter 35, a high setting load is automatically selected when a low-speed movement instruction is given from the NC machine to the movement unit, and a low setting load is automatically selected when a high-speed movement instruction is given. Further, a method of gradually or continuously changing a low setting load in response to an instructed movement speed e is also proposed. Accordingly, it is possible to reduce damage of the work or the machine when the movement unit collides with the work or the other member of the machine. In particular, it is possible to reduce damage in the event of a collision when the movement unit moves at a high speed.